RFC 7858 explicitly defines an out-of-band key pinning profile as one
authentication mechanism. It uses the same format for representing
the pin as HPKP does (RFC 7469).
By logging this pin directly upon first use of the X.509 credentials,
we make it a little bit easier for an admin to publish part of a
pinset.
For ideal operation (including preparation for key rollover), a backup
public key should also be provided, but this is not defined
functionally here.